Various visibility conditions typify many applications requiring deployment of a camera for remote vision. Such a camera is often a video camera that continuously provides images of a scene. The camera is often required to operate around the clock—during day and night, and under changing weather conditions. The camera may be stationary. In many cases the camera is portable or carried by a vehicle, for terrestrial or airborne tasks, and as such can be exposed to unpredictable or rapidly changing visibility circumstances. A good example is a camera employed for Head Mounted Display (“HMD”). HMD concerns a helmet or goggles or monocle wearer, within or outside a vehicle, whether for military or civilian purposes. HMD prevalently features image projection reflected from monocle or goggles lenses or a helmet visor to the eyes of the wearer in registration with the direct scene as seen by the wearer through the visor or the goggles. HMD users can include a pilot or another airplane crew member, a vehicle operator (in space, air, sea, or land), an arms operator, a foot soldier, and the like. For the sake of simplicity, reference below shall be frequently made to the example of a helmet visor and a pilot, whereby it is noted that the principles concerned are well applicable to other devices and methods with analogous implications.
In airplane cockpits, Heads Up Display (“HUD”) is giving way to HMD. The image often includes a vision enhancement display, wherein the field of view as seen by the user through the visor, namely a direct scene, is combined with an image of the same view made to reflect from the visor. The combination is conducted in registration, namely—the projected image of the field of view converges, in real time, with that of the actual direct scene view as seen by the user through the visor. The projected image is captured by a camera, and is manipulated by image processing means available onboard (often on the helmet). If necessary, with the aid of positioning means, the system can calculate in real time the user's head (such as the pilot's helmet) position and view orientation, and provide the image compatible to the user's field of view. If the camera is mounted on the helmet or attached to the user's head or on a suitable headset or goggles, the burden of calculating the relative fields of view of the user and the camera can be relieved or spared entirely, as the camera can be aligned to face the field of view of the user. The installment of a camera on a helmet or another eyepiece support calls for miniaturization of the camera in size and weight as much as possible, so as to eliminate interference to the user. A compact camera has limited room and poses a difficulty for containing space-consuming features, such as night vision enhancement, spectral conversion and high quality broad spectrum perception. The use of a single sensor for various wavebands, such as visible spectrum (for daylight vision), NIR (for night vision), or infrared (such as for thermal detection), imposes a difficult task for the sensor to achieve. A single sensor cannot adapt simultaneously for optimal detection of different wavelength ranges and/or wide range of illumination levels without limiting resolution and refresh rates. The use of separate cameras, one for each waveband or illumination level, incurs the addition of excess weight when several cameras are used simultaneously. Alternatively, repeated switching between different cameras is cumbersome, and increases the manufacturing and maintenance costs.